1. Field of the Invention
The present invention relates generally to a V-type engine equipped with a supercharging device, and more particularly, to an engine having an engine body formed to have a couple of V-shaped cylinder banks and an engine-driven supercharger driven to rotate by a crank shaft disposed in the engine body through an endless belt engaged with both of the engine-driven supercharger and the crank shaft.
2. Description of the Prior Art
There has been proposed a V-type engine equipped with an engine-driven supercharger, in which a camshaft is disposed in common for driving inlet valves and exhaust valves in each of a couple of cylinder banks formed in the V-shape as parts of an engine body and the engine-driven supercharger is disposed on the engine body between the cylinder banks to be driven by a crank shaft disposed in the engine body through an endless belt engaged with both of the engine-driven supercharger and the crank shaft, as disclosed in the Japanese utility model application published before examination under publication number 61-1624. In the case where the V-type engine thus constituted is provided with a distributor driven to rotate directly by the camshaft, it is desired that the distributor is mounted on the cylinder bank disposed on the tension side of an endless belt which is engaged with both of one end portion of the camshaft provided therein and one end portion of the crank shaft for rotating the camshaft because the distributor is required to rotate in exact synchronism with revolutions of the crank shaft.
In the event of such a V-type engine as described above, it is feared that antiknock property is degraded due to intake air compressed by the engine-driven supercharger and therefore a compression ratio in each of cylinders in the cylinder bank may be set to be relatively low for avoiding the degradation of antiknock property. However, the low compression ratio in the cylinder results in a disadvantage that thermal efficiency in the cylinder is reduced so as to bring about deterioration in fuel consumption.
In view of this, with the intention of suppressing knocking in the cylinder without being attended with the disadvantage mentioned above, it has been proposed to provide a valve timing control mechanism for changing the opening timing of each of the inlet or exhaust valves and cause the same to so operate that an overlapping duration in which both the inlet and exhaust valves are kept open is made longer in accordance with increase of engine speed. If the V-type engine is of the double overhead camshaft (DOHC) type with a pair of camshafts provided in each of cylinder banks formed in the V-shape for driving respectively inlet valves and exhaust valves in the cylinder bank, the valve timing control mechanism is constituted in the form of a device for varying the relative phase of the revolution of the camshaft for driving the inlet or exhaust valves in relation to the revolution of a pulley mounted on the same camshaft to engage with an endless belt from a crank shaft. With such a valve timing control mechanism, although it is possible to make the overlapping duration longer by advancing a time point at which the inlet valve is opened in accordance with increase of engine speed or by retarding a time point at which the exhaust valve is closed, it is desired that the valve timing control mechanism is attached to the camshaft for driving the exhaust valves so that the time point at which the exhaust valve is closed is retarded to make the overlapping duration longer for the reason that the operation of the engine is kept stable even in a condition of light load, such as an idling condition.
From the above point of view, in the case of the DOHC V-type engine equipped with the engine-driven supercharger, it is desired that both the distributor and the valve timing control mechanism are provided in the cylinder bank which is disposed on the tension side of the endless belt engaged with each of the camshafts and the crank shaft and according to this desire the distributor and the valve timing control mechanism are mounted respectively on one end portion of the camshaft for driving the inlet valves and one end portion of the camshaft for driving the exhaust valves provided in the cylinder bank which is disposed on the tension side of the endless belt engaged with each of the camshafts and the crank shaft.
In the case where the DOHC V-type engine equipped with the engine-driven supercharger, which is provided with the distributor and the valve timing control mechanism mounted respectively on one end portion of the camshaft for driving the inlet valves and one end portion of the camshaft for driving the exhaust valves provided in the cylinder bank which is disposed on the tension side of the endless belt engaged with each of the camshafts and the crank shaft as described above, is transversely mounted on a vehicle in such a manner that the crank shaft disposed in the engine body extends along the direction of the width of the vehicle, if the engine-driven supercharger is disposed on the engine body between the cylinder banks formed in the V-shape or on the right or left side of the engine body, the engine-driven supercharger projects in a relatively large way from the upper end or the right or left end of the engine body and therefore an engine room of the vehicle, in which the DOHC V-type engine equipped with the engine-driven supercharger is contained, is caused to be insufficient in space around the engine body. Accordingly, for restricting the engine-driven supercharger from projecting in a relatively large way from the engine body, it is considered that the engine-driven supercharger is mounted on the engine body to jut out backward or forward, namely, widthwise from one of the cylinder banks. However, if the engine-driven supercharger is mounted on the engine body to jut out widthwise from the cylinder bank which is disposed on the slack side of the endless belt engaged with each of the camshafts in the same cylinder bank and the crank shaft, the engine body is apt to lean undesirably so as to bring down the cylinder bank which is disposed on the slack side of the endless belt engaged with each of the camshafts in the same cylinder bank and the crank shaft and thereby to bring about undesirable rolling in response to the revolutions of the crank shaft when the engine starts operating.